1. Field of the Disclosure
The disclosure relates to a power supply technique. Particularly, the disclosure relates to a power supply system with a variable supply voltage.
2. Description of Related Art
With development of technology, electronic products with various functions are gradually developed, and the electronic products with various functions not only satisfy diverse needs of people, but are also widely used in people's daily life and make people's life more convenient.
FIG. 1 is a circuit block diagram of an existing power supply system. Referring to FIG. 1, the power supply system 100 includes a power adaptor 110, a switching circuit 120, a charger 130, a battery set 140 and a main equipment 150. The power adaptor 110 generates a power voltage Vin. The power voltage Vin can be supplied to the main equipment 150 and/or can charge the battery set 140 through the charger 130. The power supply system 100 selects the power adaptor 110 or the battery set 140 for powering the main equipment 150 through the switching circuit 120. Therefore, the switching 120 can provide a supply voltage Vdcb for supplying power to various DC/DC transformation units in the main equipment 150. For example, a DC/DC transformation unit 154 outputs a core voltage Vcore and supplies the core voltage Vcore to a central processing unit (CPU) for utilization. A DC/DC transformation unit 152 outputs a 1.5V voltage to a chipset, and a DC/DC transformation unit 156 outputs a 3.3V voltage to a keyboard controller.
The power voltage Vin generated by the power adaptor 110 is a fixed voltage. In order to match a charging voltage level required by the battery set 140 and the charger 130, the fixed power voltage Vin has to be higher than the charging voltage level. Generally, the power voltage Vin generated by the power adaptor 110 is 19V. When the switching circuit 120 selects the power adaptor 110 for powering the main equipment 150, the supply voltage Vdcb of the switching circuit 120 is also 19V. Since voltage differences between inputs and outputs of the DC/DC transformation units 152-156 in the main equipment 150 are relatively great, power conversion efficiency is not ideal. For example, the power voltage Vin is 19V, so that the supply voltage Vdcb is also about 19V, and the voltage differences between inputs and outputs of the DC/DC transformation units 152 and 156 are respectively 17.5V and 15.5V. Generally the DC/DC transformation units 152-156 respectively have a buck structure, and when the voltage difference of the buck DC/DC transformation unit is excessively large, the power conversion efficiency is not good.
Generally, when the power conversion efficiency is improved according to the conventional technique, an advanced power adaptor is used to achieve higher power conversion efficiency. However, the design of the advanced power adaptor may greatly increase fabrication cost.
The power adaptor is generally used in the electronic products, and energy loss is inevitable after multiple conversions of the supply power. Many countries in the world participate in the Energy Star project promoted by U.S. Environmental Protection Agency. The Energy Star project is an energy conservation project for consumable electronic products, and a purpose thereof is to reduce energy consumption. Therefore, the electronic products manufactured by manufactures have to satisfy certain specifications in order to label an Energy Star tag thereon.
Since the power voltage Vin of the power converter 110 is a fixed voltage and cannot be decreased, it is of no avail for power management of the whole system. Therefore, it is an important issue to develop an adjustable power voltage to achieve a power supply system with a variable supply voltage to reduce energy loss during the power conversion process.